75 research outputs found
Knocking on New Physics' door with a Scalar Resonance
We speculate about the origin of the recent excess at ~750 GeV in diphoton
resonance searches observed by the ATLAS and CMS experiments using the first 13
TeV data. Its interpretation as a new scalar resonance produced in gluon fusion
and decaying to photons is consistent with all relevant exclusion bounds from
the 8 TeV LHC run. We provide a simple phenomenological framework to
parametrize the properties of the new resonance and show in a model-independent
way that, if the scalar is produced in gluon fusion, additional new colored and
charged particles are required. Finally, we discuss some interpretations in
various concrete setups, such as a singlet (pseudo-) scalar, composite Higgs,
and the MSSM.Comment: 20 pages, 4 figures, 1 table. Extraction of the ATLAS 13 TeV diphoton
signal and 8 TeV ZZ bound corrected, no sizable change in the final
combination. Presentation improved, references added, and conclusions
unchanged. Version to appear in EPJ
Implications of the discovery of a Higgs boson with a mass of 125 GeV
The rather precise knowledge of the mass of the Higgs boson and of its
couplings has important consequences for the physical phenomena taking place at
the Fermi scale. We analyze some of these implications in the most motivated
frameworks for physics at that energies - supersymmetry, models of a composite
Higgs boson, and the Standard Model itself. At the same time, precision
experiments in flavour physics require a highly non-generic structure of
flavour and CP transitions. In any model of electroweak symmetry breaking with
a relatively low scale of new phenomena, as motivated by naturalness, some
mechanism has to be found in order to keep unwanted flavour effects under
control. We devote particular attention to the consequences of the approximate
U(2)^3 symmetry exhibited by the quarks of the Standard Model. The combined
analysis of the indirect constraints from flavour, Higgs and electroweak
physics will allow us to outline a picture of some most natural models of
physics near the Fermi scale. Although non trivially, a few models emerge that
look capable of accommodating a 125 GeV Higgs boson, consistently with all the
other constraints, with new particles in an interesting range for discovery at
the LHC, as well as associated flavour signals. This is particularly
interesting in view of the forthcoming improvements in the direct experimental
investigation of that energies. Finally, the measurement of the last parameter
of the Standard Model - the Higgs quartic coupling - has important consequences
even if no new physics is present close to the Fermi scale: its near-critical
value, which puts the electroweak vacuum in a metastable state close to a phase
transition, may have an interesting connection with Planck-scale physics. We
derive the bound for electroweak vacuum stability with full two-loop precision,
and explore some possible implication of near-criticality.Comment: PhD thesis. 223 pages, 46 figures, 24 table
and in Simplified New Physics Models
The decays and , being the
theoretically cleanest rare decays of mesons, are very sensitive probes of New
Physics. In view of the excellent prospects of reaching the Standard Model
sensitivity for by the NA62 experiment at CERN and for
by the KOTO experiment at J-PARC, we study them in the
simplest extensions of the SM in which stringent correlations between these two
decays and other flavour observables are present. We first consider simple
models with tree-level Z and Z' contributions in which either MFV or a
symmetry is imposed on the quark flavour-violating couplings. We then compare
the resulting correlations with those present in generic models in which the
latter couplings are arbitrary, subject to constraints from
processes, electroweak and collider data. Of particular interest are the
correlations with and which limit the
size of NP contributions to and
, depending on the Dirac structure of couplings and the
relevant operators. But in MFV also the constraint from
turns out to be important. We take into account the recent results from lattice
QCD and large N approach that indicate in the SM to be
significantly below the data. While in many models the enhancement of
implies a suppression of , we
present two models in which these observables can be simultaneously enhanced
relative to SM predictions. A correlation between and
, found by us in the simple models considered here,
should be of interest for NA62 and LHCb experimentalists at CERN in the coming
years. The one with will be tested at Belle II.Comment: 32 pages, 6 figures, 1 table. v2: updated analysis in section 4,
matches version published in JHE
Toward a coherent solution of diphoton and flavor anomalies
We propose a coherent explanation for the 750 GeV diphoton anomaly and the
hints of deviations from Lepton Flavor Universality in B decays in terms a new
strongly interacting sector with vectorlike confinement. The diphoton excess
arises from the decay of one of the pseudo-Nambu-Goldstone bosons of the new
sector, while the flavor anomalies are a manifestation of the exchange of the
corresponding vector resonances (with masses in the 1.5-2.5 TeV range). We
provide explicit examples (with detailed particle content and group structure)
of the new sector, discussing both the low-energy flavor-physics phenomenology
and the signatures at high . We show that specific models can provide an
excellent fit to all available data. A key feature of all realizations is a
sizable broad excess in the tails of invariant mass
distribution in , that should be accessible at the LHC
in the near future.Comment: v2: 32 pages, 9 figures, 2 tables. Published version. Extended
discussion about the flavor structure of the model and high-PT phenomenology,
typos corrected. Added note about the relevance of the paper in light of the
absence of the diphoton signal at the LH
B-physics anomalies: a guide to combined explanations
Motivated by additional experimental hints of Lepton Flavour Universality
violation in B decays, both in charged- and in neutral-current processes, we
analyse the ingredients necessary to provide a combined description of these
phenomena. By means of an Effective Field Theory (EFT) approach, based on the
hypothesis of New Physics coupled predominantly to the third generation of
left-handed quarks and leptons, we show how this is possible. We demonstrate,
in particular, how to solve the problems posed by electroweak precision tests
and direct searches with a rather natural choice of model parameters, within
the context of a flavour symmetry. We further
exemplify the general EFT findings by means of simplified models with explicit
mediators in the TeV range: coloured scalar or vector leptoquarks and
colour-less vectors. Among these, the case of an -singlet vector
leptoquark emerges as a particularly simple and successful framework.Comment: 33 pages, 7 figures, 2 tables. Extended discussion and one plot added
on single production of leptoquarks, typos corrected, references adde
Probing Lepton Flavour Universality with decays
We analyse the rare processes in view of the recent
hints of violations of Lepton Flavour Universality (LFU) observed in B meson
decays. If, as suggested by present data, the new interactions responsible for
LFU violations couple mainly to the third generation of left-handed fermions,
decays turn out to be particularly interesting: these are
the only kaon decays with third-generation leptons (the neutrinos) in
the final state. In order to relate B-physics anomalies and K decays we adopt
an Effective Field Theory approach, assuming that the new interactions satisfy
an approximate flavour symmetry. In this framework we
show that O(1) deviations from the Standard Model predictions in branching ratios, closely correlated to similar effects in , are naturally expected. The correlation of
, , and
the LFU violations in B decays would provide a very valuable tool to shed more
light on this interesting phenomenon.Comment: 11 pages, 2 figure
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